1. Field of the Invention
The present invention relates to the preparation of silane from methyldichlorosilane and chlorosilanes, and, more especially, to the preparation of silane from methyldichlorosilane and trichlorosilane and/or tetrachlorosilane.
2. Description of the Prior Art
Silane is a preferred raw material for producing, by decomposition, silicon of a grade which may be employed particularly in semiconductor or photovoltaic devices, e.g., solar cells.
It is known to this art to prepare silane by disproportionation of trichlorosilane in the presence of various catalysts according to the following equilibrium reactions: EQU 2HSiCl.sub.3 .revreaction.H.sub.2 SiCl.sub.2 +SiCl.sub.4 ( 1) EQU 2H.sub.2 SiCl.sub.2 .revreaction.HSiCl.sub.3 +H.sub.3 SiCl (2) EQU 2H.sub.3 SiCl.revreaction.SiH.sub.4 +H.sub.2 SiCl.sub.2 ( 3)
giving, overall: EQU 2HSiCl.sub.3 .revreaction.SiH.sub.4 +3SiCl.sub.4 ( 4)
Thus, according to French Pat. No. 2,261,977, it has been proposed to prepare silane by disproportionation or redistribution of HSiCl.sub.3 in a bed of solid anion exchange resin at a temperature which is sufficient to cause the vaporization, out of the reaction zone, of the lowest boiling reaction products and to cause the condensation of the higher boiling liquid product resulting from the reaction, SiCl.sub.4, and its outflow away from the reaction zone. The temperature at the top of the bed is maintained at a level above the boiling point of SiH.sub.4 and below the boiling point of H.sub.3 SiCl, and SiH.sub.4 containing small amounts of hydrogen and chlorosilane which require a downstream separation, is recovered from the bed.
This process, which is extremely complicated and difficult to carry out, employs either a continuous distillation of the products of the various equilibrium reactions (1), (2) and (3) and permits some separation of the various products present, or employs a purification of SiH.sub.4 in a charcoal trap.
Furthermore, there are also known various processes for producing dichlorosilane by disproportionation of trichlorosilane in the presence of various catalysts. Among the known catalysts for this purpose, the following have been proposed, in particular: tertiary amines of the general formula NR'R"R'" in which R', R" and R'" are identical or different alkyl radicals (French Pat. No. 1,111,925), amine hydrochlorides (French Pat. No. 2,096,605), N-substituted pyrrolidones (French Pat. No. 2,290,447) and tetraalkylureas (French Pat. No. 2,290,448).